X-linked inheritance

X-linked inheritance describes how traits encoded on the X chromosome are passed from parents to children. In humans, females have two X chromosomes, while males have an X and a Y. Because the Y carries few genes, a male has only one copy of most X-linked genes; he is described as hemizygous, and whatever allele his single X carries is expressed.

In X-linked recessive conditions such as hemophilia A and B, Duchenne muscular dystrophy and red-green color blindness, one altered copy is enough to affect a male, while a female is usually affected only if both her copies are altered. Women with one copy are carriers, often unaffected or only mildly affected, partly because one X chromosome is randomly switched off in each of their cells. A carrier mother passes the allele to half of her sons, who are affected, and to half of her daughters, who become carriers. A father never transmits an X-linked trait to a son, who receives his Y instead, but every daughter of an affected father inherits his X.

In rarer X-linked dominant conditions, one altered copy causes the condition in either sex, often more mildly in females. Pedigrees showing many affected males and no father-to-son transmission point to X-linkage, first described by Thomas Hunt Morgan in 1910 for white eyes in fruit flies.